This invention relates to information recording media in the form of light-readable discs, and, more particularly, to an improved design for such media which, among other benefits, will tolerate greater error in the manufacturing process and permit the use of less expensive optical reading equipment.
Such light-readable information recording media in the form of discs are well known, as shown, for example, in Kramer U.S. Pat. No. 5,068,846.
Commercially available audio compact discs ("CDs") and compact disc read-only memories ("CD-ROMs") are examples of recording media of this general type. Recent interest has also been shown in using media of this type for recording other kinds of information, such as movies or other similar real-time audio/visual programming.
Storage of such information in digital form requires enormous capacity. In addition to the storage of data itself, the recording medium must also provide for error correction, synchronization and modulation of the stored data.
For example, in general commercial use, standard disc-shaped audio recording media are designed to be approximately 120 millimeters in diameter. A center hole approximately 15 millimeters in diameter is provided to accommodate the shaft of the player spindle around which the disc rotates. As a result, the area immediately adjacent the center hole must be utilized as a clamping area, and can hold no data. Space for data is therefore limited.
Because such a large amount of data must be stored in such a small area, the details of the optical storage structure must be extremely small. This leads not only to extremely tight manufacturing tolerances but also requires sensitive optical reading devices.
In cross-section, the disc-shaped, light-readable recording medium in general commercial use is approximately 1.2 millimeters thick. Although any transparent material may be used to form the predominant portion of the medium, polycarbonate plastic with a refractive index of 1.55 is typically used. Data are contained in a pit and land structure impressed along the top surface of this transparent material. This structure is covered by a very thin, adherent, metal reflective layer which is conductive in nature, typically aluminum, of approximately 70 nanometers thickness. Atop this is placed a protective layer of approximately 10 to 30 micrometers, typically lacquer, which fills the pit indentations in the reflective layer and provides a smooth, substantially planar upper surface for the medium upon which labelling or other information may be placed.
The optical structure of the medium is read by a laser beam which is configured below the medium, in typical use operating at a wavelength of 780 nanometers in air and focussed at the reflective layer. The laser beam passes through the bottom of the transparent material and through the optical structure of pits and lands (which is seen by the laser from below as a series of bumps and lands), and is reflected off the reflective layer, through the transparent material and out of the medium to an optical reading structure.
It is generally accepted that the means by which the data are read (and, hence, the means by which the difference between bumps and lands is seen by the optical reader) is through utilization of destructive interference caused by diffraction effects. This has led to the standardization of pit depth/bump height (depending upon the plane of reference) in commercial light-readable information recording media at just less than approximately one-quarter of the wavelength of the laser light within the transparent material. Light from a laser operating at 780 nanometers would have a wavelength within the polycarbonate material of the media (R.I.=1.55) of approximately 503 nanometers. For tracking reasons well known to those skilled in the art, pit depth/bump height has been standardized at just less than a perfect one-quarter wavelength, or 126 nanometers. This is reflected in the recommendation of Philips, the assignee of the Kramer patent, that the compact disc pit depth/bump height be maintained at approximately 0.12 microns.
The reasoning behind a standard of approximately one-quarter wavelength is set forth in K. Pohlmann, The Compact Disc Handbook, 2d ed. (1992), an accepted reference on CD design. At pages 55-56, Prof. Pohlmann describes the generally accepted view of the operation of the light-readable information recording system in which the pit depth/bump height is preferably maintained near one-quarter of the wavelength of the light:
"Light striking land thus travels at a distance one-half the wavelength (one-quarter plus one-quarter) further than light striking a bump. This creates a phase difference of one-half wavelength between the part of the beam reflected from the bump and the part reflected from the surrounding land . . . . The phase difference causes the two parts of the beam to destructively interfere with and cancel each other, forming a diffractive pattern. In short, a bump disperses light, reducing the intensity of the reflected light."
This reduced intensity stands in sharp contrast to the reflection of virtually all of the light from the land, enabling the optical reader beneath the medium to detect the point of changeover from bump to land. Indeed, the greater the intensity difference, the easier it becomes for the optical reader to detect the changeover. Thus, Philips recommends a pit depth equal to one-quarter wavelength to achieve maximum diffraction efficiency.
In light of this accepted view of the method of operation of a light-readable information recording system, it stands to reason that a light-readable disc utilizing a pit depth/bump height of one-half the wavelength of the laser light being used would be inoperable. Viewed again from the perspective of an optical reader beneath the medium, light striking land would travel one full wavelength (one-half plus one-half) farther than light striking a bump. This would place the part of the beam reflected from the bump and the part reflected from the surrounding land in perfect phase, yielding no destructive interference and, consequently, no difference in light intensity. In such event, an optical reader would be completely unable to differentiate between bumps and lands and thereby incapable of retrieving any data.
Unexpectedly and completely contrary to the conventional view, it has been determined that, by designing the pit depth/bump height to be approximately one-half of the wavelength of the laser light source, an improved light-readable recording disc can be achieved. Not only has such a disc been found to be operable, but it has been determined that the intensity difference between the bump and land areas detected by an optical reader is actually greater than for the one-quarter wavelength pit previously thought to be optimal.
In view of the foregoing, it is an object of this invention to provide a light-readable information recording disc that is easier to read, which can thereby reduce the cost and complication of optical reading devices.
It is another object of this invention to provide an improved light-readable information recording disc which, by virtue of the stronger signal intensity difference that it generates, will provide for greater tolerance in the manufacturing of such discs.